Waves in Communication

by Ryan Moorcroft

Sometimes people say hello to each other by waving. However, we can communicate with each other using many different types of waves, not just the wave of our hands. The first example that probably comes to your mind is sound waves. We can produce sound when air passes through our vocal cords, causing them to vibrate. These vibrations propagate through the air and eventually into our eardrums. There is a limit to how far a sound wave can travel as the wave will dampen, meaning the wave will sound quieter with distance from the speaker.

Until a couple of centuries ago, long-distance communications were slow, unreliable, and inefficient. Nearly all messages had to be carried physically. That's why we developed new technologies to help communicate using waves in the electromagnetic spectrum to overcome this problem. Some of these new communication technologies are even useful in short ranges. This article will help you to understand the applications of waves in communication and the physics involved!

Electromagnetic Waves in Communications

Below is a diagram of the electromagnetic spectrum; it shows the entire distribution of electromagnetic radiation according to increasing frequency or increasing wavelength. Waves with higher frequencies, such as gamma rays and x-rays have the smallest wavelength and the highest energies of electromagnetic waves. While microwaves and radio waves possess the lowest frequencies and energies, but the largest wavelengths.

The different sections of the electromagnetic spectrum are ordered by frequency and wavelength, Wikimedia Commons CC BY 3.0

Electromagnetic waves have many uses within the realms of communication. Different parts of the electromagnetic (EM) spectrum are suitable for different applications. The different energies, frequencies, and wavelengths associated with each separate part of the EM spectrum give advantages and disadvantages depending on their use. We only utilise the low-frequency parts of the electromagnetic spectrum in communications, EM radiation with higher frequencies than visible light becomes impractical for various reasons.

Radio Waves in Communications

Radio waves have the longest wavelengths and lowest frequencies in the electromagnetic spectrum. Most radio wave frequencies are essentially transparent to the atmosphere, so are very useful when communicating long distances. They also pass straight through the human body, so aren't harmful at all. The radio spectrum itself is very broad. It ranges from wavelengths between 1 mm and 10,000 km, which is larger than the radius of the Earth! So radio waves of different wavelengths and frequencies will exhibit very different behaviours. Television and radio broadcasts are typical examples of how these waves are applied in communications.

We can transmit and receive radio waves using oscillations in electrical circuits. When a conductor absorbs a radio wave it generates an alternating current. The frequency of the alternating current matches the frequency of the radio wave. This is how we encode information electronically before radio transmission and decode it after the wave is received.

Visible Light Waves in Communications

Of course, we already use visible light to communicate naturally with each other, so we will discuss visible light purely as communications technology. In 1792 a Frenchman named Claude Chappe invented the semaphore telegraph system. The semaphore was a primitive way of transmitting information long-distance using visual signals. Tall towers were built within line of sight of each other, at distances between$5$and$20$miles depending on terrain. There would be a human operator at each tower with a spyglass, watching their neighbouring towers for signals. After receiving a message the operator would then relay the message to the next tower along. This was the precursor to the electrical telegraph, which would replace the semaphore less than a century later.

In the modern era, we use fibre optic cables to transmit signals as visible light. Fibre optic cables can be used to deliver high-speed internet. Fibre optic cables are made of flexible glass fibre, which allows electromagnetic waves to be internally reflected and therefore travel through the cable at the speed of light. These cables are safely enclosed by a plastic sheathing to protect the delicate glass fibres inside. Optical fibre cables can be used to transmit signals very long distances, as there is only a small loss of the signal due to scattering or absorption of the wave when it is reflected at the boundaries of the cable. The better quality glass allows for smaller losses as the wave travels through the cable.

A visible light ray travelling through a fibre optic cable via internal reflection, adapted from image by Chris Woodford CC BY 3.0

Infrared Waves in Communications

Infrared light encompasses all electromagnetic radiation between$700\mathrm{nm}$($0.0007\mathrm{mm}$) and$1\mathrm{mm}$. Infrared transmissions are mostly used in short-range communications. Your TV remote control is a common example of this. The main problem with infrared communications is that the waves will not pass through solid objects and are partially absorbed by the atmosphere, giving it a short-range. In some contexts (like the comfort of your own home) this can be turned into an advantage! An infrared TV remote in the living room will not interfere with similar devices in other rooms in the house because the signals cannot travel through walls. Furthermore, infrared transmitters are relatively cheap to make and have low power requirements, which keeps costs low for consumers.

Satellite communications

One way of communicating on a global scale is to utilise satellites, often employed by satellite television companies and mobile phone networks. For instance, a mobile phone tower will transmit and receive data from many nearby mobile phones on the ground via radio waves. The mobile phone tower will then communicate with a satellite in geostationary orbit using microwaves. A geostationary satellite will remain stationary above a specific point of the Earth's surface at all times, so that communication with ground-based equipment can occur uninterrupted, 24/7. The microwave spectrum is used as it's not blocked by the Earth's atmosphere and can transmit more data per second than radio waves, because of their higher frequencies.

The satellite can then communicate with other mobile phone towers within a large area on the Earth's surface. Commercial satellites very rarely directly communicate with each other, signals must be relayed to receivers on the ground.

Using a satellite to communicate over long distances, Wikimedia Commons CC BY 3.0